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Chapter 8: Chemical Reactions Mrs. Wanstreet The Nature of Chemical Reactions Chemical reactions: Occur when substances undergo chemical changes to form new substances Have reactants and products Reactants: substance that undergoes a chemical change Products: substance that is the result of a chemical change Energy and Reactions Energy must be added to break bonds There are several types of energy: What bonds? Light Thermal (heat) Kinetic (motion) Energy is transferred to the compound to break bonds and rearrange the molecules Forming New Chemical Bonds Releases Energy! The formation of carbon dioxide and water from isooctane and oxygen produces the energy used to power engines! As the bonds of isooctane break down, energy is released as the new compounds are formed The energy is in the form of heat This is a Combustion Reaction! Energy is Conserved in Chemical Reactions Energy released by a reaction is stored as chemical energy This is stored in the reactants Chemical energy: energy stored within atoms and molecules that can be released when they react Types of Reactions There are seven types of reactions to know: Exothermic Endothermic Synthesis Decomposition Combustion Single-Displacement Double-Displacement Exothermic Reaction Exothermic: energy transferred from the reactants to the surroundings Heat and light are the most common forms of energy given off Examples: Explosions Bioluminescence Endothermic Reactions Endothermic: energy transferred to the reactants from the surroundings Usually in the form of heat The result is cold on the outside of the beaker Synthesis and Decomposition Reactions Synthesis Reaction: two or more substances combine to form a new substance A + B AB 2H2 + O2 2H2O Decomposition Reaction: one substance breaks down, or decomposes, into two or more simpler substances AB A + B 2H2O 2H2 + O2 Single and Double Displacement Reactions Single-Displacement Reaction: occurs when one element replaces another in a compound A + BC AC + B Zn + Cu(NO3)2 Zn(NO3)2 + Cu Double-Displacement Reaction: occurs when elements switch partners AB + CD AD + CB AgNO3 + NaCl AgCl + NaNO3 Precipitates A precipitate is usually formed in a doubledisplacement reaction Precipitate: an insoluble compound formed during a reaction It comes out of solution as a solid! Precipitation Reactions: Precipitate of Sulfur White substance is the silver chloride precipitate Combustion Reactions Combustion of Methanol Not all combustion reactions release CO2 and water (Ex.: the combustion of magnesium metal) Reduction-Oxidation Reactions “Redox” Reactions A reaction that occurs when electrons are transferred from one reactant to another Rusting is an example of a redox reaction Iron turns to Fe2O3 (rust) when exposed to the environment (O2) Each iron atom (Fe) loses 3e- and each oxygen atom (O) gains 2e- Oxidation of Iron Balancing Chemical Equations The Law of Conservation of Mass: the mass of what you start with has to be equal to the mass of what you end with This is why equations must be balanced!!! A balanced equation says the number of atoms you started with has to be equal to the number of atoms you end with Describing a Chemical Reaction Pb(NO3)2 + 2KI PbI2 + 2KNO3 How many potassium iodides are in the reactants side? How many nitrate molecules are in the product side? The reactant side? Pb(NO3)2 + 2KI PbI2 + 2KNO3 How would you say it in words? Lead (II) nitrate reacts with 2 Potassium iodides to produce Lead (II) iodide and 2 Potassium nitrates Balancing Equations A chemical equation is the written format of a reaction All chemical equations MUST BE BALANCED!!! Balanced equation has the same number of atoms of each element on both sides of the equation YOU CAN ONLY ADD COEFFICIENTS TO BALANCE! You CANNOT change subscripts!!! Remember coefficients are the big numbers, subscripts are the little numbers below the element Balancing Equations: Example 1 Is the equation balanced? Count the number of EACH TYPE OF ELEMENT ON BOTH SIDES OF THE EUQATION Balancing Rules: Step 1: describe the reaction in words Step 2: write a chemical equation for the reaction using chemical formulas Step 3: Check the equation to see if there are balanced atoms on both sides Make sure the reactants are on the left and the products are on the right Draw a line between the reactants and products, write out each element, count the number of atoms of each element on both sides Step 4: If it isn’t balanced then choose coefficients (big numbers) that balance the equation Balancing Equations Example 2 Silver Nitrate reacts with Sodium Chloride to produce Silver Chloride and Sodium Nitrate. Write the equation!!! AgNO3 + NaCl AgCl + NaNO3 Check to see if the equation is balanced?! YES!!!!! Try These: Balance these equations: NaBr + Cl2 NaCl + Br2 2NaBr + Cl2 2NaCl + Br2 Al(OH)3 + CO2 Al(HCO3)3 Al(OH)3 + 3CO2 Al(HCO3)3 Mg + O2 MgO 2Mg + O2 2MgO More Practice! 2NO2 N2O4 Already Balanced! NaHCO3 H2O + CO2 + Na2CO3 2NaHCO3 H2O + CO2 + Na2CO3 NaCl + H2SO4 Na2SO4 + HCl 2NaCl + H2SO4 Na2SO4 + 2HCl